Small Wonders of Biology

One of the central dogmas of molecular biology has been that proteins, the cellular workhorses of living organisms, play a crucial role in controlling gene activity and that a class of RNA molecules called the messenger RNA(mRNA), translate instructions from genes to create proteins. The activity of these mRNA molecules have been thought to be regulated by a novel class of small RNA molecules called siRNAs (short interfering RNA) through the process of mRNA degradation - a phenomenon named as RNA interference(RNAi) or gene silencing.

But a series of recent discoveries is presenting a different scenario altogether. A groundbreaking research in C. elegans some 15 years back, has lead to the discovery of a yet another class of naturally occurring small RNA, called microRNA that appear to be crucial in regulating gene expression reveals that siRNAs are not the sole mediators of gene silencing mechanism in a cell.

MicroRNAs

MicroRNAs are naturally occurring and highly conserved small noncoding RNAs of approximately 21 nucleotides that regulate mRNA expression by binding to the 3'-untranslated regions (3'-UTR) of mRNAs. This halts the production of proteins - a function opposite to that of mRNAs. This is a naturally occurring gene silencing mechanism that is triggering much interest in the same field.

MicroRNAs so called due to their tiny size, appear to control a third of our genes and are implicated in a wide range of biological processes as diverse as early development, cell proliferation, cell death, apoptosis, fat metabolism, cell differentiation, and in cancer development such as in chronic lymphocytic leukemia, colonic adenocarcinoma, and Burkitt's lymphoma. They have also been implicated in brain development and certain brain disorders, development of diabetes and also in viral infections such as HIV.

Though the initial focus of researchers was mainly on the double stranded synthetic siRNA molecules that mediate RNA interference, the micro RNAs have gained tremendous impetus from all directions in the last five years, in the form of RNAi companies and academic research institutions expanding their research into miRNA related products for both therapeutic and diagnostic applications.

The burgeoning interest in this new field could be realized by way of increasing number of scientific papers published in 2001 till date from just four papers in 2001 to as high as 700 in 2006. Nearly 500 papers have been published in 2007 alone with many more to come in the next few months.

Evidences are showing the possible roles of each miRNA and the cellular processes they control. It is now a proven fact that miRNAs control over one third of the human genome and each miRNA may even regulate multiple genes. Hundreds of miRNAs have been identified through bioinformatics and the discovery that miRNA control only the translation of messenger RNAs (mRNAs) and not their stability, further explains the major reason behind the mismatch between mRNA expression profiles and protein expression profiles. It is estimated that the number of clinically important miRNAs could be at least 1000 and researchers view miRNA as a much more exciting concept than RNAi because of its natural occurrence unlike the manmade RNAi phenomenon.

These tiniest entities in the human genome which once were of microimportance have now become things of macroimportance today, emerging as useful research and diagnostic tools and as potentially important biomarkers of diseases and as potential therapeutic molecules. The advent of new and efficient technologies such as microarrays, and locked nucleic acid technologies, has led to manifold advances in miRNA research to the extent of proving with evidence that there is a link between miRNAs and diseases such as cancer.

Noteworthy research in miRNA, include the discovery of an miRNA that regulates brain development in zebra fish by researchers at the New York University School of Medicine, the discovery of an miRNA related to blood cell development and blood cancers such as leukemia by Stanford University researchers; and the discovery of an miRNA with HIV resistance in AIDS patients by Japanese researchers.

In the last few years, scientists have unearthed unique miRNA "expression signatures" associated with different types of cancers, infectious diseases in humans, and diabetes in mice and have also proven that miRNA expression profiles serve as diagnostic and prognostic markers for lung cancer. These provide a basis for further development.

Molecular diagnostics

Of all potential applications of miRNA, molecular diagnostics seems to be catching up rapidly as these small wonders are emerging as disease biomarkers. For decades together the mRNAs and proteins were the ruling class of biomarkers in the field of molecular diagnostics. But the overwhelming number of mRNAs (approximately 22,000) and the complexity of proteins have been presenting significant challenges complicating the process of screening the entire human genome and in systems biology for researchers.

But the relatively less number of miRNAs in the human body when compared to the manifold number of mRNAs and proteins have now increased the possibility of screening the entire genome and in genetic diseases especially cancer miRNAs are emerging as preferred class of biomarkers to mRNAs and proteins. Hence, instead of resorting to mRNA expression profiling researchers are turning toward miRNA expression profiling for diagnosing several types of human cancers and other disease states. Their small size which initially proved to be a hindrance for their detection is now proving to be a short cut and effective solution for miRNA expression profiling thus aiding in easing the currently complicated field of cancer diagnostics.

Apart from cancer, their involvement in the regulation of genes involved in other processes such as brain function and immune function can lead to diagnostics for the early detection of neurodegenerative and autoimmune diseases. With the discovery of viral miRNAs, detection of viral infections is yet another potential application. The last two years especially have witnessed a phenomenal increase in the knowledge about the biogenesis and functional aspects of miRNA and as a result miRNA has now emerged as yet another gene knockdown technology for research, therapeutic and diagnostic applications.

MicroRNA therapeutics is yet to emerge but they are also of potential therapeutic value. MiRNA therapeutics could be classified as a kind of antisense RNA therapy targeted against RNAs- a way of switching off the expression of disease associated protein. This is mediated through an antisense drug that blocks or destroys the corresponding mRNA that is responsible for the protein's synthesis. Here miRNAs are expected to serve as breakthrough molecules that aid the discovery of next generation antisense drugs.

Companies such as Rosetta Genomics, Santaris Pharma, and Alnylam are employing various strategies to counter various miRNAs that have been implicated in diseases such as cancer. Alternatively, it may also be possible to control phenotypes such as tumor growth, insulin secretion, and fat metabolism through therapeutic alteration of microRNA levels.



The future
In future, miRNAs could be used for phenotype analysis and some researchers have also exploited the synthetic hairpin structures of miRNAs for studying the disease pathways, mechanism of action of drugs and also to decipher which genes are affected in the disease process. Information about the existence of miRNAs with unique functions has created intriguing possibilities for researchers to get started on to in-depth functional miRNA research. Here advanced microarray technologies are expected to provide useful insights about the role of each miRNA in regulating gene expression and about the role of miRNA in specific disease states so as to identify novel biomarkers and therapeutic targets.

Prognostics are yet another emerging application of miRNAs as they can aid in the prediction of patient response to radiotherapy of certain cancers and thus in patient selection for clinical studies and overall prognosis of the disease.

Success with SiRNA therapeutics is said to be a major driving force for miRNA therapeutics. As for diagnostics, it's difficult to find a disease indication that's not linked to miRNA at this point, so it's only a matter of time and validation before miRNA diagnostics appear in the clinic. The molecular diagnostics market continues to grow, both by new applications and into existing diagnostic markets such as hematology, cytogenetics, and conventional pathology and the inherent attributes of miRNAs are making them an ideal class of diagnostic analytes.

MiRNA research is relatively new with exciting discoveries and is in a growth and discovery phase, it is up to researchers and biotechnology companies to capitalize on the initial path breaking discoveries in the field of miRNA and a huge market opportunity is now available for those wanting to develop high-through put and cost effective analytical tools for miRNA research.

Researchers and companies have started embracing RNAi as a novel tool for analyzing gene function and identifying new drug targets. Target validation and RNAi-based therapeutics have their own advantages such as being more personalized forms of therapies compared to traditional treatments. Here, the emerging technology of miRNA and the proven technology of RNAi could be thought of as mutual drivers for each other.